Field of the Invention
The present invention lies in the field of manufacturing disc-like plastic objects, for instance information carriers such as CDs. A two-fold injection moulding unit is known for this purpose. This injection moulding unit comprises two injection moulds coupled mechanically to each other. Via a multiple hot-runner beam the unit is fixed rigidly against a front plate by means of bolts such that the injection buses debouching with a passage in the respective mould cavities are enclosed between the two, such that the central hole of a disc-like object is formed by retracting a punch. This requires an expensive and time-consuming construction, wherein movable wedges are placed behind the punch to hold the punch in place when it is loaded with a high injection pressure during the injection of plastic.
It is comparatively simple to form the central hole in a disc by means of a punch. The drawback hereof is that this requires an injection bush having to be moved aside for the purpose, which is awkward because an injection bush is connected by means of bolts to said hot runner beam. This combination will thus have to be able to move during punching and be in fixed position during injection. The injection bushes are guided in a guide formed by a front block. This block is cooled from 120.degree. C. to 50.degree. C. The hot runner temperature amount to about 350.degree. C. As a consequence the hot runner therefore expands much more than the front block and, because the injection bushes are fixed to the hot runner with bolts, they will be clamped fixedly on one side by expansion of the hot runner. It is an object of the invention to provide a solution to this problem. A further object of the invention is to provide an injection moulding unit enabling a simpler and less expensive structure.